In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-recording layer corresponding to the image area, removing the unnecessary image-recording layer corresponding to the non-image area by dissolving with an alkaline developer or a developer containing an organic solvent thereby revealing the hydrophilic surface of support.
In the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-recording layer by dissolving, for example, with a developer is required. However, it is one of the subjects to simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the alkali development processing of high pH has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, as one means for the simplification, it has been strongly desired to conduct the development with a nearly neutral aqueous solution.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image-outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate technique of carrying digitalized image information on highly converging radiation, for example, a laser beam and conducting scanning exposure of a lithographic printing plate precursor with the radiation thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of the important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
In the case where the simplified plate making operation as described above is carried out under printing environment, an image-recording layer after imagewise exposure is likely to be fogged before development. Therefore, an image-recording layer capable of being handled in a bright room or under a yellow lump and a light source are necessary.
As such a laser light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 and a solid laser, for example, YAG laser, are extremely useful because these lasers having a large output and a small size are inexpensively available. Also, an UV laser can be used.
Based on the background described above, adaptation of plate making operation to both simplification and digitalization has been demanded strongly more and more than ever before.
In response to such a demand, for instance, in European Patent 1,342,568, a plate making method is described in which a lithographic printing plate precursor having an image-forming layer containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder provided on a hydrophilic support is developed with a gum solution. According to the plate making method, the lithographic printing plate precursor is exposed imagewise using an infrared laser to fuse the hydrophobic thermoplastic polymer particles, thereby forming image and then the unexposed area is removed with the gum solution to perform development.
However, the method of developing the lithographic printing plate precursor using the image formation by heat fusion of fine particles with a single unit of the gum solution has problems in that since the fine particles in the unexposed area have good water dispersibility, the gum solution is colored and fatigued in a short time and that since the component removed with development is hardly eliminated, for example, by a filter, the component removed with development reattaches to the surface of printing plate after the development processing to cause ink stain.
In WO 05/111727, a processing method of a lithographic printing plate precursor is described in which a lithographic printing plate precursor comprising (i) a hydrophilic support and (ii) a photosensitive layer which contains a radically polymerizable ethylenically unsaturated monomer, a radical polymerization initiator and an infrared absorbing dye, is capable of being cured with infrared laser exposure and is capable of being developed with a gum solution is exposed imagewise with an infrared laser and the uncured area of the photosensitive layer is removed with a single unit of a gum solution. Also, in U.S. Pat. No. 6,902,865 and JP-A-2006-106700 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) (corresponding to US2006/0046199A1), a developing method of a lithographic printing plate precursor is described in which a radical polymerization type photosensitive layer is cured with infrared laser exposure and the unexposed area is removed with an aqueous developer having pH of 2 to 10. Further, in EP-A-1,788,444, a developing method of a lithographic printing plate precursor is described in which a radical polymerization type photosensitive layer is cured with infrared laser exposure and subjected to gumming treatment with continuous two bath construction.
However, these techniques still have the problem in that the component removed with development reattaches to the surface of printing plate after the development processing to cause ink stain.